This application claims the priority of German Application No. 199 49 258.1, filed Oct. 13, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a brake system for motor vehicles and, more particularly, to a brake system for motor vehicles having a defining unit for determining the driver""s desire to brake, an observation unit for determining the desired vehicle handling and the actual vehicle handling, and a central controller which, as a function of the output signals of the defining unit and of the observation unit, determines desired wheel quantities for the wheel-selective controlling of the individual brake actuators of the vehicle wheels.
A brake system of this type is known, for example, from German Patent Document DE 197 42 988. This known brake system includes a brake at each of its four wheels, which is actuated by one electrically controlled brake actuator or control element respectively. The brakes are actuated, on the one hand, when the driver of the motor vehicle makes a braking torque demand by way of a brake pedal unit and, on the other hand, when additional brake torque demands are present because of sensor signals of additional braking functions (such as the TCS (Traction Control System) or the DSC (Driving Stability Control).
The driver""s desire to brake is determined by a defining unit which consists of a system of three computers and several sensors which are mounted on the brake pedal. As a result of the multiple redundancy, an error-tolerant design of the basic brake is achieved. By analyzing additional driving dynamics sensors, the desired vehicle handling as well as the actual vehicle handling is determined in an observation unit. Thus, so-called higher brake functions are implemented. In this case, the observation unit may be integrated in the defining unit. If the higher brake functions corresponding to the prior art are designed to be free of errors, these can also be distributed to fewer than three computers.
The optional wheel-selective operation of the brake actuators or the control elements is carried out by a central controllerxe2x80x94the so-called central controlxe2x80x94as a function of the desired vehicle handling and of the actual vehicle handling. In this case, the observation unit and the defining unit may be integrated in the central controller.
Furthermore, non-prior art brake control systems are also known by the assignee of the present invention that have not yet been published. In these, decentralized force controllers are provided which are mounted on each wheel. By means of these brake control systems, the brake force control performs a cascade control, on the one hand, by the overriding central controller and, on the other hand, by the subordinate force controllers. Because, in this case, only desired forces are to be transmitted to the brake actuators, the control of the wheel slip continues to take place in the central controller. For this purpose, for example, by way of a communication network, first the actual wheel speeds must be exchanged; then the control quantities must be calculated; and these must then be sent back to the wheel units. The additional data transmission time adds up to idle control time and impairs the control quality of the slip control. This, in turn, significantly influences the braking distance, the vehicle stability, as well as the comfort.
It is an object of the invention to improve a brake system of the above-mentioned type such that the control quality of the brake system is increased, the idling control time is reduced, and the safety of the vehicle is thereby improved.
This object is achieved by brake system for motor vehicles having a defining unit for determining the driver""s desire to brake, an observation unit for determining the desired vehicle handling and the actual vehicle handling, and a central controller which, as a function of the output signals of the defining unit and of the observation unit, determines desired wheel quantities for the wheel-selective controlling of the individual brake actuators of the vehicle wheels. The desired wheel quantities are the desired wheel force (Fdesired i) and the desired wheel slip (Sdesired i) A force-slip controller (8i) which is mounted on the vehicle wheel (10i), is connected in front of each brake actuator (9i). The force-slip controller (8i) receives as input signals at least the desired wheel force (Fdesired i) from the central controller (3), the desired wheel slip (Sdesired i) from the central controller (3), the wheel reference speed from the central controller (3), the measured actual wheel force (Factual i) directly and the measured actual wheel slip (Sactual i) directly or the measured actual rotational wheel speed, for calculating the actual wheel slip (Sactual i). As a function of at least one operating parameter, the brake system carries out either a wheel slip control or a wheel force control. Advantageous further developments of the invention are described herein.
The brake system for motor vehicles according to the invention has a defining unit for determining the driver""s desire to brake, an observation unit for determining the desired vehicle handling and the actual vehicle handling, and a central controller which, as a function of the output signals of the defining unit and of the observation unit, determines desired wheel quantities for the wheel-selective controlling of the individual brake actuators of the vehicle wheels. The desired wheel quantities are the desired wheel force and the desired wheel slip. A force-slip controller mounted on the vehicle wheel is connected in front of each brake actuator or control element. For calculating the actual wheel slip, a wheel reference speed is provided to the force-slip controller by the central controller. This wheel reference speed largely corresponds to the speed of the free-rolling wheel. In the simplest case, this is the vehicle reference speed which is formed in the central controller.
Each force-slip controller receives as input signals at least the desired wheel force from the central controller, the desired wheel slip from the central controller, the wheel reference speed from the central controller, the measured actual wheel force, and the measured actual wheel slip or the measured actual wheel speed for determining the actual wheel slip. As a function of at least one operating parameter, the force-slip controller carries out either a wheel slip control or a wheel force control.
Preferably, the wheel force control is carried out instead of a wheel slip control when the vehicle speed falls below a defined threshold value and/or when the desired wheel slip falls below a defined threshold value.
This introduction according to the invention of a cascade control with a subordinate slip and force controller in the wheel units is such that the transmission behavior of the brake actuator is modeled exclusively in the pertaining wheel unit.
As a result of the invention, improved slip control quality is achieved because of shorter idle control times by a faster reversing when entering slips. Furthermore, the components can be optimally assembled because, first, the functionality and the software of the central controller is not influenced by the used actuators and, secondly, the combination of different actuators (hydraulic, electromechanical) is possible. Thirdly, the physically oriented interface is always very suitable for linking an entire vehicle controller to subordinate control devices in a distributed control system. Furthermore, there is a very good independent evaluation possibility and comparability of various brake actuators and their control quality in the overall system. Finally, for increasing the availability, it is possible to prevent locking as a wheel-type fall-back plane.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.